Twin hull variable draft drilling vessel

ABSTRACT

THE VESSEL COMPRISES A PAIR OF LATERALLY SPACED ELONGATED HULLS HAVING A PLURALITY OF UPSTANDING COLUMNS SPACED THEREALONG SUPPORTING A WORKING PLATFORM IN SPACED RELATION ABOVE THE HULLS A DISTANCE SLIGHTLY GREATER THAN THE MAXIMUM ANTICIPATED WAVE HEIGHT. THE HULLS BUOYANTLY SUPPORT THE VESSEL IN A LOW DRAFT FLOATING CONDITION WITH THE HULLS HAVING FREEBOARD. THE HULLS HAVE BALLAST COMPARTMENTS TO SUBMERGE THE HULLS AND PORTIONS OF THE STABILIZING COLUMNS TO A DISTANCE OF APPROXIMATELY HALF THE   EFFECTIVE HEIGHT OF THE STABILIZING COLUMNS WHICH IS SLIGHTLY GREATER THAN MAXIMUM ANTICIPATED WAVE HEIGHT, TO MAINTAIN THE VESSEL IN A HIGH DRAFT FLOATING CONDITION WITH THE PLATFORM ELEVATED ABOVE THE WATERLINE. THE COLUMNS STABILIZE THE VESSEL IN THE HIGH DRAFT CONDITION ABOUT ROLL AND PITCH AXES. THE WORKING PLATFORM MOUNTS EITHER A DRILLING RIG OR A HEAVY DUTY CRANE OR LIKE OPERATIONAL EQUIPMENT ALONG THE CENTERLINE OF THE VESSEL.

1971 s. H. LLOYD m TWIN HULL, VARIABLE DRAFT DRILLING VESSEL 5Sheets-Sheet 1 Filed Sept. 4, 1968 SAMUEL HARRY LLOYD III BY 1 MMATTORNEYS 1971 s. H. LLOYD lll TWIN HULL, VARIABLE DRAFT DRILLING VESSELFiled Sept. 4, 1968 5 Sheets-Shoot 2 INVENTOR SAMUEL HARRY LLOYD I11 BYAIM.

ATTORNEYS Nov. 2, 1971 s. H. LLOYD III 3,616,773

TWIN HULL, VARIABLE DRAFT DRILLING VESSEL Filed Sept. 4, 1968 5Sheets-Sheet 5 INVENTOR SAMUEL HARRY LLOYDHI BY a/ mi ATTORNEYS Nov. 2,1971 s. H. LLOYD m 3,616,773

TWIN HULL, VARIABLE DRAFT DRILLING VESSEL Filed Sept. 4, 1968 5Sheets-Sheet 4 FIG. '7

INVENTOR 8 SAMUEL HARRY LLOYDllI iw m ATTORNEYS United States Patent@fice 3,616,773 Patented Nov. 2., 1971 US. Cl. 114-5 84 Claims ABSTRACTOF THE DISCLOSURE The vessel comprises a pair of laterally spacedelongated hulls having a plurality of upstanding columns spacedtherealong supporting a working platform in spaced relation above thehulls a distance slightly greater than the maximum anticipated waveheight. The hulls buoyantly support the vessel in a low draft floatingcondition with the hulls having freeboard. The hulls have ballastcompartments to submerge the hulls and portions of the stabilizingcolumns to a distance of approximately half the effective height of thestabilizing columns which is slightly greater than maximum anticipatedwave height, to maintain the vessel in a high draft floating conditionwith the platform elevated above the waterline. The columns stabilizethe vessel in the high draft condition about roll and pitch axes. Theworking platform mounts either a drilling rig or a heavy duty crane orlike operational equipment along the centerline of the vessel.

This application is a continuation-in-part of application Ser. No.666,395, filed Sept. 8, 1967 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a variable draftvessel having twin hulls for use with a drilling platform and other deckload and which is adapted for deep water drilling and other operations.

In attempts to locate new oil fields, an increasing and significantquantity of well drilling has been conducted in offshore sea and likelocations where a substantial body of water overlies the oil field. Thishas generated considerable interest and effort in offshore and deepwater drilling. One current method of offshore drilling utilizes a fixeddrill platform mounted on legs resting on or driven into the sea floor.These, however, are feasible for use only in relatively shallow depthsof water normally not greater than about 300 feet, which is a realisticdepth limit for practical commercial operations. Deep water drilling hasheretofore been accomplished with the employment of specificallydesigned and constructed vessels or rigs which have certain inherentdisadvantages and limitations noted hereinafter. A brief review of bothoffshore and deep water drilling methods heretofore practiced and thevessels or rigs employed therewith will provide a more clearappreciation and understanding of the present invention, as well as aclear distinction between those vessels or rigs employed in offshoredrilling and those employed in deep water drilling.

An early method of offshore drilling, still currently employed, providesfor the erection of a self-contained fixed platform which is supportedby pilings driven into the sea floor and has a drilling rig, auxiliaryequipment and crews quarters located on the platform. At the conclusionof the drilling, a tender is brought to dismantle and remove thedrilling equipment and, in the case of a dry hole, the entireself-contained platform is dismantled and removed by tender. A variationof the foregoing method provides a somewhat smaller platform similarlyerected on piles and having a drilling rig located thereon while theauxiliaries, equipment and crew are located in a tender tied alongside.At the conclusion of the drilling, the platform is likewise either leftfor oil production or is dismantled and removed in the case of a dryhole. Another method employs a self-elevating barge which is towed tothe drilling site and provided with columns or legs which are thenlowered and embedded in the sea floor. The barge is then jacked up so asto clear the water surface and serves as a platform on which thedrilling rig, crew, and auxiliary equipment are subsequently positionedfor drilling operation. At the conclusion of the drilling, in the eventof a producing well, a fixed platform generally is erected forcontinuing oil production and the self-elevating barge is towed toanother drilling site. The foregoing methods are each feasible for usein relatively shallow water depths of normally 300 feet or less. Thefactors governing construction and operation of the vessels or rigsutilizing any of the foregoing arrangements are not significantlyconcerned with stability and other problems involved in conducting adrilling operation from a floating platform, as in deep water drilling,since the abovediscussed rigs and vessels are constructed for sea bottomengagement.

Deep water and exploratory drilling has been accomplished by means ofsurface floating drilling vessels which are either towed orself-propelled to the drilling site and are self-contained in that thedrilling rig, auxiliary equipment and crews quarters form an integralpart of the vessel. These floating drilling vessels are anchored overthe drilling site and are normally provided with a central openingthrough which the drilling rig is operated. Drilling operations fromthese floating vessels are, however, highly restricted by sea stateconditions, since excessive vessel motion in heave, pitch and roll canand does damage the drilling equipment as well as aggravate the problemof maintaining the vessel anchored directly over the drilling site. Thestability characteristics of such a single-hulled drilling vessel areaccordingly not conducive to efficient oil drilling operations. Acatamaran type oil drilling vessel has been constructed; however, whilethat type vessel affords greater stability than a single-hulled vessel,it involves substantial problems which include excessive vessel motiondue to wave action, no substantial motion minimiz ing characteristics,overstability and resultant snapping action which tosses personnel aboutand may endanger the drilling string and other equipment. Accordingly,while these vessels are not geographically limited to offshore drillingoperations, they are limited to use in restricted or calm waters.

Deep water and exploratory drilling operations have also employedsemisubmersible platforms which, like the floating vessels, arecompletely self-contained. In this latter type, the platform issupported on a plurality of structural members including stabilizingcolumns joined at their lower ends to a base flotation structure which,when in unballasted condition, floats the entire structure above thesurface of the water with the base structure having freeboard. Afterbeing towed in freeboard condition to the drilling site, the baseflotation structure is deballasted to submerge the same. To maintain thedrilling platform above water in a relatively stabilized condition, theplatform support structure includes a plurality of columns which extendbetween the platform and the base structure and are partially submergedto the extent that the displacement of such columns in conjunction withany residual displacement of the base structure supports the overallsemisubmerged structure.

In one type of these prior semisubmersible platforms, the stabilizingpatterns of the stabilizing columns form substantially symmetricalequilateral polygons with the columns located at the apices of therespective polygons which are normally square or triangular in shape.The equilateral symmetrical polygonal arrangement provides substantiallyidentical righting moments about the roll and pitch axes, as well as anyintermediate axes, regard less of wave direction. Another form ofsimilar semisubmersible vessel includes a plurality of stabilizingcolumns interconnected adjacent their upper ends so that only thecolumns float in the water, with the columns again being arranged in asymmetrical equilateral polygon. In general, while such prior vessels ofthis type provide adequate stability for well drilling operations, theyhave an inherent disadvantage of very low mobility bewteen drill ingsites due to the shape and frontal area of the polygonally arrangedcolumns and/or base structure presented to the water surface when suchvessels are towed; their towing speed often does not exceed 2 knots.Additionally, to achieve the necessary displacement for supporting thefull weight of drilling rig, deck load, etc., for such a vessel having asymmetrical equilateral polygonal configuration of base structureandstabilizing columns, such prior type vessels are of such largedimension that they cannot pass through narrow waterways like the Panamaand Suez Canals, whereby their use is accordingly restricted.

Accordingly, it is a primary object of this invention to provide a newimproved twin hull variable draft vessel which minimizes above-discussedand other shortcomings of prior offshore and deep water drilling rigsand vessels, and provides various advantages in construction, mode ofoperation and result over prior systems.

It is another object of the present invention to provide a variabledraft vessel which, particularly in column stabilized floatingcondition, has the characteristic of minimizing vessel motion due toexcitation forces caused by wave action (hereafter called motionminimizing characteristic). It is a related object to provide such avessel affording improved motion minimizing characteristics in vesselpitch, roll and heave.

It is a related object of the present invention to provide a variabledraft column stabilized vessel having rapid mobility in transit.

It is still another object of the present invention to provide a twinhull variable draft column stabilized vessel which is operable in greatdepths of water as a selfcontained drilling vessel.

It is a further object of the present invention to provide a variabledraft vessel having twin hulls which has a drilling platform mounted ona deck which is above the hulls an effective height slightly greaterthan the maximum anticipated wave height.

It is still a further object of the present invention to provide avariable draft drilling vessel having stabilizing columns configured toreduce displacement of the vessel in heave in the high draft floatingcondition due to the inertia thereof and to provide reduced pitch androll angles.

It is still a further object of the present invention to provide a deepwater drilling vessel having twin hulls the draft of which is variableby ballasting and deballasting of the twin hulls to selectively orientthe vessel in a low draft floating condition with the hulls havingfreeboard and a high draft column stabilized floating condition,

It is a related object of the present invention to provide a variabledraft deep water drilling vessel having twin hulls including separateballast compartments in each hull together with ballasting means forselectively ballasting and deballasting the several compartments in eachhull thereby selectively correcting the heel and trim angles.

It is another object of the present invention to provide as improvedvariable draft vessel usable as a tender for other vessels, or as aplatform for a heavy duty crane, dredge or other equipment, as well asfor a drill rig.

4 These 'and other related objects and advantages of the presentinvention will become more apparent from the following specification,claims, and appended drawings wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of thetwin hull drilling platform and tender barge or vessel according to thepresent invention, with the vessel illustrated in the low draft towingcondition;

FIG. 2 is a side elevational view of the vessel similar to that of FIG.1 and illustrating the vessel in the high draft, column stabilized,floating condition;

FIG. 3 is a plan view of the vessel, with portions broken out for easeof illustration;

FIG. 4 is a horizontal sectional view taken about on line 4-4 of FIG. 1;

FIG. 5 is a transverse sectional view taken about on line 55 of FIG. 4;

FIG. 6 is a transverse sectional view taken about on line 6-6 of FIG. 4;

FIG. 7 is a bow end elevational view of the vessel in the columnstabilized high draft condition;

FIG. 8 is an aft end elevational view of the vessel in the columnstabilized high draft condition;

FIG. 9 is a schematic view of one of the hulls of the vesselillustrating the ballast system therefor; and

FIGS. 10A and 10B are elevational and plan diagrammatic illustrationsrespectively of another embodiment of the vessel hereof.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawings, thereis shown a variable draft drilling platform (hereinafter referred to asa vessel) generally indicated at 10 comprising a pair of transverselyspaced, elongated hulls 12 extending in spaced parallel relation andproviding sufficient displacement to support vessel 10 in the low draftfloating condition with the hulls having freeboard indicated at f. Hulls12 are substantially identical one with the other, each hull having asubstantially rectangular cross section as seen in FIGS. 5 and 6. Eachhull 12 has designed hydrostatic properties and hydrodynamiccharacteristics, including being streamlined for minimum resistance inmotion. Each hull has freeboard f and a bow 14 and stern 16 with midbodybetween. Inwardly curved outboard side formations at bow portions 14 areindicated at 18. Hulls 12 are thus formed substantially streamlined inshape to minimize resistance to towing through the water when vessel 10is entirely supported by hulls 12 in the low draft floating condition asillustrated in FIG. 1.

A platform or main deck 20 is supported in spaced relation above hulls12 a predetermined height h (FIG. 1) which is at least equal to andpreferably slightly greater than the maximum anticipated wave height,i.e., the vertical distance between wave crest and trough, by supportstructure comprising a plurality of longitudinally spaced, transverselyextending truss formations generally indicated at 22 and a plurality ofspaced stabilizing columns 24. The first, second, third and fifth trussformations 22, looking aft in FIG. 4, each includes a lower transverselyextending base tubular member 26 having opposite end portions overlyingand secured to hulls 12 to maintain the latter in predetermined spacedrelation preferably a distance at least equal to a hull beam. Thelengths of the members 26 of the first, third, and fifth trussformations 22 are identical while the base member 26 of the second trussformation is extended to the longitudinal centerline of the centrallylocated stabilizing columns 24. A plurality of diagonally and upwardlyextending columns 28 are suitably secured at their lower ends to basemembers 26 and at their upper ends to platform 20 to form a pair oftransversely extending V-shaped truss formations as best seen in FIGS.

5, 6, and 8. The fourth truss formation 22 comprises a pair of V-shapedformations without a lower base member. The first, third, and fourthtruss formations 22 have outboard columns 30 joined at their lower endsto the out-board edges of hulls 12 and which extend upwardly andinwardly to the outboard edges of platform 20. A plurality of inboardcolumns 32 extend from the inner edges of hulls 12 upwardly tointermediate portions of platform 20 at longitudinally spaced positionsalong the bulls between transverse truss formations 22 to provideadditional support for platform 20. It will be noted that theforwardmost truss formation 22 is spaced a considerable distance aft ofthe forward ends of hulls 12 to provide a substantial open area 33,unencumbered by support structure, between forward portions of thebulls. It will be apparent to one skilled in the art in light of thedisclosure herein that other specific support constructions and columnand truss arrangements can be used in a vessel according to thisinvention.

As discussed more fully hereinafter, the support structure also includesstabilizing columns 24 extending upwardly from the upper surface ofhulls 12, a distance which may be equal to and is preferably at leastslightly greater than the maximum anticipated wave height, the verticaldistance between wave crest and trough. In the preferred illustratedembodiment, the columns 24 extend upwardly to platform 20 and form apart of the structural support therefor whereby their effective heightis equal to the height h between the upper surface of hulls 12 and theunderside of platform 20. In the preferred embodiment, columns 24 arelocated adjacent opposite ends of each hull 12 with a third column beingprovided on each hull intermediate the ends thereof. As seen in sectionin FIG. 4, columns 24 preferably are generally oval shaped withlongitudinally elongated vertical sides and cylindrical fore and aftvertical end sections 34. Besides providing streamlining in thedirection of the vessels longitudinal axis, this provides increasedvessel stability. Use of columns 24 provides better motion minimizingcharacteristics when the vessel is in the floating high draft condition.Stabilizing columns 24 are preferably constant in cross sectional areathroughout their effective height h. It will be understood that eitheror both the upper and lower ends of the columns may be reduced in crosssection, for example, to form frusto-conical sections, to providemechanical connection between the columns and the hulls and platformwhich do not substantially affect the effective height or make thelatter subject thereto.

A drilling platform 35 is spaced above main deck 20 adjacent the forwardend of the vessel by a plurality of support beams 36 secured at theirlower ends to platform 20. A house 38 is provided adjacent the forwardend of drill platform 35 adjacent the forward edge thereof. Trussformations 40 have a pair of transversely spaced legs 42 secured attheir lower ends to mounting members 44 fixed to platform 35 adjacentthe aft edge thereof, legs 42 providing the aft support for trusses 40.There is supported on drill platform 35, a drill rig or derrick 46,having a pair of transversely spaced base truss formations 48 formingdiverging legs which are pivotally secured at their lower ends tomounting members 44, whereby drill rig or derrick 46 can be pivotallymoved between a vertically extending drill operating position seen inFIG. 2, and a lowered inoperative position seen in FIG. 1. The baseportion 48 of derrick 46 seats against and is suitably secured by meansnot shown to truss formations 40 to maintain the same in a verticalposition whereby a drill string 50 can be supported from the upper end52 of the derrick 46 so that string 50 extends between hulls 12 on thecenter line of the vessel forward of the first truss formation 22 intoopen area 33. Main deck 20 has a deck superstructure 54 supporting aweather deck 56 mounting a pair of transversely spaced, upstanding beams58 carrying a transverse beam 60 at the upper ends thereof forsupporting the upper end of derrick 46 when the latter is pivoted to thelowered horizontal position shown in FIG. 1. The main decksuperstructure 54 houses the machinery, crews quarters, and additionaldrilling equipment while weather deck 56 supports an after deckhouse 62and a plurality of transversely extending pipe racks 64. Alongitudinally extending catwalk 66 is positioned intermediate piperacks 64 to provide access to any portion along the racks. The top deckof deckhouse 62 supports a heliport 70 and ballast control house 71.

In a preferred form hereof, particularly as seen in FIGS. 4, 7, and 8,columns are disposed along the outboard sides of hulls 12 such that theinner faces of columns 24 lie parallel to and in vertical alignment withthe center line of the associated hull. The displacement and stabilityrequirements of columns 24 are such that these columns have a transversedimension locating an outboard portion 72 of each column 24 inoverhanging relation to the outer hull sides, with the longitudinal axesof columns 24 being spaced laterally outwardly of the center line of thehulls and located inwardly of and adjacent the outer hull sides. Thecentroids of the water plane areas defined by the cross sections of thecolumns 24 are thus located an extended distance from the center line ofthe vessel on opposite sides thereof to develop larger moments ofinertia of the water plane areas about the roll axis than wouldotherwise be the case if the longitudinal center lines of each of thehulls and their associated stabilizing columns were coincident. Theupper surfaces of the outer overhanging portions 72 of columns 24 form acontinuation of main deck or platform 20 as seen in FIGS. 3 and 7 andeach such upper surface adjacent the corners of the vessel mounts a pairof mooring pulleys 76,, which forms a part of the vessel positioning orstation keeping system. A pair of mooring winches 78 are located in amachinery space adjacent the bottom of each of the four cornerstabilizing columns 24 and carry anchoring cables 80 which extend overpulleys 76 through chocks 84. Winches 78, pulleys 76, and cables 80provide an 8-point mooring to anchors system which serves to hold andmaintain the vessel in fixed position over the drilling site when thevessel is floating in the high draft column stabilized condition. Othertypes of positioning or station keeping devices, such as dynamicpositioning devices, mooring to piles and the like, may be employed withequal facility and the foregoing described anchoring system isrepresentative only of a preferred form thereof.

A pair of cranes 84 and 86 are mounted on opposite sides of the vesseland may be of any conventional design, including the usual booms 88 andoperating cabs 90. Cranes 84 and 86 may have any desired capacity, forexample, 50 tons, crane 84 having an 80 foot boom and crane 86 having afoot boom. The smaller crane 84 is preferably employed to service theself-contained drilling rig aboard vessel 10 such as for example raisingor lowering the drill rig or loading and unloading pipes from pipe racks64. The heavier crane 86 is preferably employed when vessel 10 isutilized as a tender, for example, in erecting or dismantling andgenerally servicing other drilling rigs. Crane 84 is pivotally mountedon a supporting column member 92 fixed at its lower end adjacent theoutboard side of one of the hulls while crane 86 is pivotally supportedon -a columnar member 94 spaced from intermediate column 24 and fixed atits lower end to the outboard side of the other hull 12.

As seen in FIG. 9, hulls 12 are each divided into compartments 96forming a plurality of ballast chambers for varying the draft of thevessel. While only the starboard hull and ballast system therefor areillustrated in FIG. 9, it will be understood that the port hull issimilarly arranged and ballasted but is of the opposite hand. Ballastchambers 96 are selectively and independently ballasted and deballastedwhereby the vessel may be partially submerged with the platform 20remaining substantially level throughout the partial submergence of thevessel and the stability of the vessel in both roll and pitch may becorrected during partial submergence and retention of the vessel at thecolumn stabilized, high draft, drilling depth. To this end, a pluralityof conduits 98 extend from a centrally located pump room P in each ofthe hulls in opposite longitudinal directions to the several ballastcompartments 96, there being, in the preferred form, four ballastcompartments in each of the aft and forward portions of each hull. Apair of conduits 100 extend aft from pump room P and terminate in a pairof compartments 102 which may be employed as supplemental ballastcompartments or as compartments containing drilling water. While not apart of the ballasting system per se, a pair of bilge water conduits 104extend fore and aft from pump room P into the bilges of the hulls andare in communication with the ballast pumps in a manner to be described.

The pump room is provided with a sea suction inlet indicated at 106 andan overboard discharge indicated at 108 controlled by suitable poweroperated gate valves 110 and 1112, respectively, the hull sides beingindicated by the dashed lines in FIG. 9. A pair of pumps 114 and 116 areprovided to suction sea water through inlet 106, past valve 110, throughpumps 114 and .116 via conduits 118 and 120 respectively, past checkvalves 122 and 124, respectively, and into a conduit 126 communicatingwith main ballast conduit 128. Opposite ends of main conduit 128communicate with fore and aft ballast conduits 98 through suitable poweroperated valves 130, ballast con-.

duits 98 being arranged in parallel at opposite ends of main conduit128. With valves 110, 122, 124, and 130 open, the four ballastcompartments at the fore and aft portions of each hull may be ballastedwith sea Water at an equal rate to maintain the platform substantiallylevel when the vessel is being partially submerged to a high draftcolumn stabilized drilling level or position.

To refloat the vessel with the hulls 12 having freeboard, valve 110 isclosed and valves 112, 130, and 132 are opened. Pumps 114 and 116operate to pump water in the same direction as before and accordinglysuction main conduit 128 through conduit 1134., thereby suctioningballast conduits 98 and withdrawing ballast water therefrom fordischarge through conduit 126, open valve 112 and outlet 108. It is thusreadily seen that by selective operation of valves 110, 112, and 130,and 132, selected compartments of compartments 96 may be ballasted anddeballasted as desired to affect the attitude of the vessel about heeland trim axes, and to assist in the drilling operation. Moreover, thiscan be accomplished when the vessel is in any operating condition, i.e.,low draft floating with the hulls having freeboard, high draft columnstabilized floating during drilling operations or any intermediateposition during submerging or refloating operations.

Conduit 192 connects the port and starboard pumprooms and communicateswith ballast conduit 128. By activating the appropriate valves,including valve 190' in conduit 192, ballast can be transferred from onehull to the other. In the event that one pumproom becomes flooded due toextreme damage, the ballasting system. can be controlled by openingvalve 190 and controlling the ballast system with the pumps in the otherhull.

The ballast drilling water conduits 100 are also connected in parallelto the aft end of main conduit 128 through suitable valves 13 8 similarto valves 130*. Thus, the ballast drilling compartments .102 can beballasted, deballasted, and selectively ballasted and deballastedsimilarly as compartments 96 by selected operation of valves 138 and theaforementioned valves 110, 112, and 13 2.

A fresh water intake or filling conduit 140 communieating with themachinery deck via uptake conduit 142, and a fresh water suction conduit1'44, communicating with the ships service fresh water pump 146, eachcom municate with a fresh water compartment 148 located aft of the pumproom on the inboard side of the hull. A conduit 149 communicates betweenfresh water pump 146 and the machinery deck via uptake conduit 150. Apair of fuel oil suction conduits 1-52 communicate with fuel oil tank154 located forward of the pump room on the inboard side of the hull andprovides fuel oil to the machinery deck via an uptake conduit 156 bymeans of fuel pump 1'58. Drill water is pumped from the aft drillingwater compartment 162 and the ballast/ drilling compartments 102 viaconduits 164 and 166, respectively, into a main drill water conduit 167by a pump .168 which delivers the drill water via suitable valve 170 tothe machinery deck via an uptake conduit 172. A drill water conduit 174communicates with the forward drilling water compartment 176 andprovides drill water to the machinery deck via pump 168 and conduit 172.Suitable valves 178 are provided in conduits 164 and 166 and thesetogether with valve 176 in conduit .174 are selectively operable to tilland suction compartments 102, 162 and 176, whereby drill water may betransferred to and from the drilling rig and may be employed for thepurposes of ballasting and deballasting the vessel.

It is a significant feature of the present invention that vessel 10 canbe towed between drilling sites at speeds on the order of 10l2 knotsproviding the present vessel with a mobility heretofore unavailable inprior semisubmersible type platforms. To this end, hulls 1 2 have bow,stern and midbody designs and have a displacement when deballasted tosupport the entire weight of the vessel in a low draft floatingcondition with the hulls 12 having freeboard. In this low draft floatingcondition, it will be noted that the mast 46 is pivoted to extendhorizontally as seen in FIG. 1, thereby lowering the overall center ofgravity of the vessel, wind resistance, and excessive dynamic loading onthe mast 46. Thus, when floating with the twin hulls having freeboard,vessel 10 has the greater righting stability and decreased roll anglescharacteristic of a catamaran type vessel. It will be seen that thesupport structure for platform 20' including truss formations 22 andstabilizing columns 24, are disposed above the waterline and accordinglydo not present a frontal area to the water to offer resistance topassage therethrough. In the low draft floating condition, only twinhulls .12 displace water and the hydrodynamic characteristics of thebulls, particularly their substantially streamline shape as well as theabsence of support structure in contact with the water, minimizes theirresistance to passage through the water and permit the towing of thevessel at significantly higher speeds than theretofore possible withprior semisubmersible platforms.

When vessel 10' reaches the drilling site, the anchors (not shown) andanchor lines 80 are deployed to maintain the vessel directly over thedrilling site. Hulls 12 are then ballasted preferably by simultaneouslyballasting fore and aft compartments 96 thereby maintaining the vessellevel, to submerge the hulls below the water line with the vessel beingsubmerged to the extent that columns 24 are partially submerged forapproximately half their effective height 11, thereby locating the meanwater line above the upper surfaces of the hulls at a distance ofapproximately half the distance between platform 20 and the uppersurface of hulls 12. In this manner, the maximum anticipated wave isprevented from acting against hulls 12 and platform 20' and acts only inthe open frame area between the hulls and the platform; this reduces theadverse effect of wave action on the vessel which has excellent motionminimizing characteristics in the column stabilized high draft floatingcondition. As the vessel is partially submerged, anchor lines 80-- aremade taut by operation of Winches 78 to maintain the vessel over thedrilling site.

At the predetermined submerged depth, valves are closed and thedisplacement of the submerged portions of columns 24 and the residualdisplacement of the hulls are suflicient to maintain the vessel in thefloating high draft condition. It is a significant feature hereof thatthe foregoing vessel has optimal stability characteristics in thefloating high draft condition. To this end, the columns are designed toprovide a sufliciently large displacement of the submerged portionsthereof and this, in combination with the displacement of the submergedhull, provides suflicient overall buoyancy to support the entire weightof the vessel including the drilling rig, crews quarters, etc., in thefloating high draft condition as well as a sulficiently large waterplane area at the aforementioned depths of submergence to provide anadequate righting moment arm to return the vessel to a level position.The columns are also designed to provide a sufficiently smalldisplacement of the submerged portions thereof to preclude largeamplitudes of the vessel displacement in heave and sufficiently smallwater plane area to provide a longer period of and hence a gentle roll.The columns provide a roll sufficiently slow as to preclude tossingabout of operating personnel, equipment and machinery on platform (whichas seen in FIG. 2 are at a considerable height above the roll axis RAwhen the vessel is in the high draft condition) and a roll ratesufficiently fast to provide adequate stability about the roll axis. Thevessel attitude about heel and trim axes can be corrected by selectedballasting of compartments 96, and, if necessary, compartments 102.

Particularly, to afford motion minimizing characteristics of the vesselwithin the designed constraints imposed on the vessel, such as widthlimitations to permit passage through the Panama Canal, the operatingparameters, for example, the drilling rig, deck load, etc., the crosssectional area and configuration of the columns, the weight distributionof the vessel, and the geometry of the submerged hulls and portions ofthe hulls to platform connections specifically the submerged columnportions are such as to maintain the natural period of the vessel in thehigh draft condition at least as great as six seconds and are preferablysuch that the natural period of the vessel in the high draft conditionlies within a range of 6 to 40 seconds.

It will be noted that the primary purpose of the present variable draftvessel is to minimize vessel motion due to wave action, particularlywhen operating in the high draft column stabilized condition. Ideally,this is accomplished by submerging the vessel approximately one-half theeffective heights of columns 26 thus precluding wave action against theplatform as well as the hulls so that only the exposed columns 24 andtrusses 22 between the platform and hulls are exposed to wave action.However, even when this vessel is operating within design limits in thehigh draft column stabilized condition with the motion-minimizingcharacteristics afforded by the described vessel construction, there issome vessel response to wave action, i.e., the wave action againstcolumns 24 and the other support structure including trusses 22. Becauseof this, when the period of the waves according to existent seaconditions is close to the natural period of the vessel, there may beamplification of vessel motion which may become so excessive as tointerfere with drilling and other operations, even though the vessel issubmerged to the usual operating condition with the mean water line atapproximately one-half the effective height h of stabilizing columns 24.It is thus necessary and desirous to alter the motion of the vessel whensuch motion amplification occurs and this can be accomplished by eitherballasting or deballasting within certain predetermined limits to varythe draft of the vessel to a greater or lesser extent from the idealcolumn stabilized high draft condition which locates the mean watersurface one-half the effective height h. The maximum variation of vesseldraft from the ideal draft in column-stabilized floating condition byballasting or deballasting the vessel is, however, limited to distanceswithin a range which do not reorient the vessel to a position whereinwave action against the vessel causes excessive impact. Thus, topreclude excessive vessel motion and impact caused by the interaction ofvessel and wave motion, the maximum variation in draft in the columnstabilized floating condition, i.e., by ballasting or deballasting, fromthe ideal column-stabilized draft of one-half I1, is such that thedistance between mean water surface and either the underside of theplatform or the topside of hulls 12 is not less than 0.75 of theexistent mean wave height.

In addition to ballasting and deballasting, the natural period of thevessel in pitch and roll may be varied by redistribution of the ballastwithin the vessel. This can be accomplished through ballast transferbetween compartments, toward or away from the ships extremities, as theconditions may necessitate, i.e., transversely or longitudinally of thevessel. In this manner, all vessel motions caused by wave action can beminimized and the stability and motion minimizing characteristics thusafforded the vessel are optimum for a vessel of the foregoingconstruction.

To refloat the vessel, the anchor lines are loosened or the anchors areshipped aboard the vessel and ballast compartments 96 and/orcompartments 102 are pumped to evacuate the water therein ashereinbefore described. The combined displacement of the hulls and thesubmerged columns is sufficient to raiSe the vessel to the low draftfloating condition illustrated in FIG. 1, the stabilizing columns actingcontinuously to stabilize the vessel during refloating operations.

In either floating condition, the stability characteristics of the twinhull vessel afford use thereof as a tender with one or both of cranes 84and 86 being operable to service another drilling rig or vessel. Thevessel is selfcontained in that crews quarters, the required auxiliaryequipment, and deck load, etc., is on board and accordingly the vesselcan provide these facilities to service another drilling structure.Auxiliary equipment, crews quarters, etc. may be located within columns24 in addition to being located on platform 20.

Certain basic principles are employed in the construction of the presentvessel:

(1) A pair of elongated, laterally spaced hulls 12, having bow, sternand midbody designs and lying in substantially parallel relation, areemployed to provide greater towing speeds as well as high stability.

(2) The hulls have sufficient displacement to float the vessel with thehulls having freeboard, and the hulls are compartmented for ballastingin a predetermined volumetric relation to the stabilizing columns 24 asnoted in points (6) and (7) hereafter.

(3a) The vessel should have at least six stabilizing columns 24, withhalf of the columns being disposed on each hull on opposite sides of theroll axis RA and a first and second pair of such columns on oppositesides of the pitch axis PA (passing through the center of flotation)with the third middle pair of such columns located adjacent orintersected by the pitch axis when a total of six stabilizing columnsare used.

(312) More specifically, if an odd number of pairs of stabilizingcolumns are employed, the middle pair should be adjacent the pitch axisPA and the other pairs of columns should be disposed in equal numbers onopposite sides of the pitch axis PA and in a generally symmetricalrelation, as illustrated, for example, in FIG. 4; whereas when an evennumber of pairs of stabilizing columns are employed, the same number ofpairs are located on the opposite sides of the pitch axis PA in agenerally symmetrical relation thereto, as illustrated for example inFIGS. 10A and 10B.

(4) To stabilize the vessel, each of columns 24 should have apredetermined area which is constant in cross section throughout theeffective height thereof per point (6) below.

(5) The platform or main deck is supported above the upper surfaces ofthe hulls a predetermined height h which is at least equal to andpreferably slightly greater 1 l than the maximum anticipated waveheight, i.e., the vertical distance between wave crest and trough.

(6) The effective height of the stabilizing columns is equal to andpreferably slightly greater than the maximum anticipated wave heightfrom crest to trough such height being substantially unaffected by anyslight changes in configuration for the mechanical connection betweenthe columns and either of the hulls and platform.

(7) The stabilizing columns 24 are constructed so that their lowerportions having a height equal to one-half the effective height providea combined displacement together with the residual displacement ofballasted hulls 12 so as to float the vessel in a height draft conditionmeeting the requisites of point (6).

(8) When floating in the high draft condition, the stabilizing columnsprovide stabilizing righting moments about the roll axis RA and thepitch axis PA in proportion to their volumetric displacement, crosssectional area and their respective distances from each such axes,whereby such righting stabilizing moments are of a magnitude to maintainthe vessel within optimum limits of roll and/ or pitch inclination andperiod of roll and/or pitch for drilling operations.

(9) The vessel is ballasted to a partial submergence of approximatelyone-half the effective height of the stabilizing columns to maintain thevessel in a high draft floating condition. To minimize vessel motionamplification under such conditions when necessary, ballast isredistributed and/or the vessel is ballasted to submerge or emerge to agreater or lesser extent from the above optimum high draft conditionsuch that the distance between the mean water surface and either theunderside of the deck or top side of the hull is not less than 0.75 ofthe mean wave height.

(10) The cross sectional area and configuration of the columns, theweight distribution of the vessel, and the geometry of the submergedhulls and portions of the columns are such as to maintain the naturalperiod of the vessel in the high draft condition at least as great assix seconds and preferably such that the natural period of the vessel inthe high draft column stabilized condition lies within a range of 6 to40 seconds.

An illustrative preferred embodiment of a vessel constructed accordingto the present invention has an overall length of 270 feet at hulls 12with each hull 12 having a beam of 30 feet and inside spacing of 30 feetfrom each other, providing an overall hull beam of 90 feet. The effective height h of the stabilizing columns 24 is 24 feet. The centroidsof the columns 24 are equally spaced 41 /2 feet from the vesselslongitudinal centerline. The pairs of outer stabilizing columns 24 arespaced about 212 feet apart, with the central pair of columns 24 beingmidway between. The length and width of the columns 24 are 34 feet and23 feet respectively with the ends being formed cylindrical in shapeproviding an overall area of approximately 667.5 square feet. To providea larger righting moment about the roll axis RA, the bottles 24preferably overhang 8 feet beyond the outboard hull sides providing anoverall vessel beam of 106 feet, and, accordingly, an overalllength-to-width ratio of approximately 2 /2:1 is provided.

SUMMARY OF CONSTRUCTION AND OPERATION Thus, the present inventionprovides a twin hull, variable draft column stabilized drilling platformhaving a plurality of connecting members including spaced upstandingstabilizing columns 24 which are fixed at their lower ends to a pair oflaterally spaced, elongated parallel hulls 12 and which support adrilling platform 20 including crews quarters and machinery spaces, attheir upper ends. The. spaced hulls are compartmented to provide ballasttanks 96 which are deballasted when the vessel is towed to and fromdrilling sites to provide sufficient hull displacement to support thevessel (and its drilling rig, crews quarters and machinery spaces) inlow draft condition with the hulls having freeboard. The hullcompartments 96 provide a safety factor in the event of a collision orotherwise rupturing certain of the compartments. At the drilling site,the hulls ballast tanks 96 and tanks 102, if necessary, are ballasted tosubmerge the hulls normally to a distance about one-half the effectiveheight of stabilizing columns 24 which is about one-half the height ofthe maximum anticipated wave whereby platform 20 remains supported abovethe maximum anticipated wave height. The displacement required tosupport the platform/ barge in the high draft floating condititon isprovided by the submerged hulls and portions of the stabilizing columns24, the vessel in this condition being otherwise unsupported. Thepresent vessel is thus different from the previously discussed priortypes of self-contained fixed platform, self-elevating barges andsurface floating vessels as the present vessel operates in two distinctconditions (1) low draft floating with the hulls having freeboard and(2) high draft column stabilized floating; thus, the vessel involvesstability and structural criteria differing from the abovementionedprior type vessels. or fixed platforms. The present vessel of thisinvention is further distinguished from such prior type vessels in thatsole support in both conditions is provided by buoyancy whereby factorsfor ground supported operation are not involved.

Moreover, the present vessel is also significantly different in concept,construction and mode of operation from previously describedsemisubmersible platforms as will now be discussed. Mobility between andstability at the drilling sites are prime requisites for an effectivedrilling vessel. The vessel of this invention can be towed betweendrilling sites at speeds greatly in excess of towing speeds for priordrilling platforms and the like, due to use of twin hulls havingsuflicient displacement to locate the stabilizing columns 24 above thewaterline when the vessel is in the floating condition and hulls havingstern, bow and midbody designs characterized by hydrodynamic andhydrostatic properties. Contrary to the provision of interconnectingbase structure at the lower ends of polygonally located stabilizingcolumns as in prior art semisubmersibles, whereby the base structurecreates significant drag when those semisubmersibles are towed on thewater surface so that towing speeds are limited to about two knots, thestreamlined parallel twin hulls 12 of the present vessel areinterconnected only above the waterline when the vessel is in thefreeboard low draft floating condition thus enabling significantlygreater towing speeds of about 10- 12 knots and thereby greatly reducingthe time in transit between drilling sites. Moreover, by utilizing twinstreamlined parallel hulls 12, the overall beam of the present vessel issignificantly less than the width of prior equilateral symmetricalpolygonally shaped semisubmersible vessels, while the present vessel cansupport a corresponding weight in drilling platform, machinery, andequipment. Further, this construction enables the present vessel to betowed through narrow waterways such as the Suez and Panama Canals, whileprevious polygonally shaped semisubmersibles could not. Additionally,the twin hulls feature hereof provides high stability in the freeboardlow draft floating condition notwithstanding the relatively high locusof the center of gravity of the overall vessel necessitated by spacingthe drilling platform, machinery, and equipment at an effective heightabove the hulls based on the anticipated sea states in which the vesselwill operate, as hereinafter more fully discussed.

The ability of the present vessel to provide a stable floating platformin various wave conditions without recourse to the symmetricalequilateral polygonally shaped structures of prior semisubmersiblesheretofore thought necessary to stabilize the same in a floating columnstabilized condition, is highly significant as it permits use of theabove-described twin hull support, thus combining in a single vessel thedesirable stability characteristics of a twin hulled low draft vesselhaving freeboard and a column stabilized high draft vessel and drillingplatform. To stabilize the present vessel and its drilling platform 20in the floating high draft condition, wave action against the vesselmust be minimized as a cause of unfavorable motion characteristics. Bysubmerging the twin hulls such that the platform. and upper surfaces ofthe hulls are respectively spaced above and below the waterline adistance at least equal to one-half the maximum anticipated wave or tohalf the effective height of columns 24, wave action against the largersurface area of hulls 12 or the drilling platform is substantiallyeliminated, and waves act only against the relatively small area of opensupport structure and framework between the drilling platform and thehulls. The location, size and configuration of the vessels stabilizingcolumns 24 are effective: to maintain the vessel in a buoyant high draftcondition; to locate and maintain the drilling platform 20 and hulls 12respectively above and below the maximum wave amplitude; to precludelarge vessel motions in heave due to large column displacement and theinertial properties of the vessel; to provide a sufficient rightingmoment when unstabilized about roll and pitch axes of a magnitude tocorrect the rolling and pitching motion; and to generally minimize theeffect of the wave action against the vessel when in the floating highdraft condition.

The stabilizing columns 24 are located adjacent opposite end portions ofeach hull with an additional column located on or adjacent thetransverse axis through the center of flotation to provide moment armsabout roll and pitch axes such that the hydrodynamic forces act toestablish righting moments in the high draft condition proportional tothe volumetric displacement of the submerged portions of the stabilizingcolumns about the roll and pitch axes to locate and maintain themetacenter above the center of gravity of the vessel for all floatinghigh draft positions of the vessel. ilt may be noted at this point thatthe width of the twin hulled vessel imposes a restriction on the momentof inertia developed about the roll axis as the transverse distance ofstabilizing columns 24 along each hull from the centerline of the vesselis limited to onehalf the width of the vessel. The stabilizing columns24 are therefore provided with water plane areas sufficiently large tocompensate for the smaller moment arm. in the transverse direction, anda preferred form of the vessel has the stabilizing columns located asclose to the outboard sides of the hulls as possible, and in theillustrated embodiment portions of the columns are outboard of the hullsto further increase this righting arm.

While the preferred form of the vessel described herein provides threeor an odd number of pairs of columns, an even number of pairs of columnscan be provided, four pairs 24 thereof being illustrated in theembodiments of the present vessel shown in FIGS. lA-10B. It is seen, inthis embodiment, that a like number of pairs of columns 24 are disposedon opposite sides of the transverse pitch axis PA and in a generallysymmetrical relation thereabout, the middle pairs of columns thereofbeing spaced on opposite sides of the pitch axis PA.

The effective height of the stabilizing columns from the upper surfacesof the twin hulls to the under surface of the platform may be equal toand preferably slightly greater than the height of the maximumanticipated wave such that in the floating high draft condition, thehulls remain underwater and the platform above water for all waves. Thestabilizing columns 24 also have a shape providing a constant crosssectional area throughout their effective height, thus presentingconstant water plane areas to the water surface. Thus, submergence andemergence of stabilizing columns 24 due to wave action act to stabilizethe vessel at a gentle stabilizing rate. If the cross sectional area ofthe upper half of stabilizing columns 24 were progressively increased inthe upward direction, a shortened and undesirable roll and pitch periodcould occur with adverse effect on the drilling operation; and aprogressive decrease in the cross sectional area of the upper half ofthe stabilizing columns 24 would result in large undesirable roll andpitch angles.

The magnitude of the cross sectional areas and hence the water planeareas and displacement of the present stabilizing columns is determinedin part by the magnitude of the displacement of the hulls andsatisfaction of other counter-vailing criteria. A sufliciently largecolumn or bottle displacement is provided to afford adequate buoyancy tosupport the vessel in the high draft condition with the hulls ballastedand providing a small proportion of total displacement. Sufficient waterplane areas are provided for the columns to insure an adequate rightingmoment about roll and pitch axes with the roll and pitch angles notexceeding optimum limits. The column or bottle displacement, however, issufficiently small so that large vessel displacements in heave areavoided. Additionally, the water plane areas are small enough to providea period of roll which provides safe and comfortable operatingconditions. The reduced water plane area also generally minimizes thewave action against the vessel.

A crane 84 is located adjacent one side of the raised platform toservice the self-contained drilling rig. A second crane 86 is preferablyalso provided on the opposite side of the vessel for like purpose.

The twin hull stability of the present vessel is great enough to enablethe vessel to carry heavy deck loads. When employed as a tender barge,one of the small cranes 84 or 86 will be eliminated and a heavy dutycrane (e.g., ton lift capacity) would be substituted; this crane wouldpreferably be mounted on one of the center columns 24 to utilize thecolumn for structural support. In this usage, not only does ballastingthe hulls submerge the vessel to provide a stable platform for drillingoperations but also the hull compartments 96 can be selectivelyballasted to correct the vessel heel and trim, respectively, to offsetthe weight of, and variations in, loads applied to the vessel via thecrane. Further, the ballast correction can be applied to the vessel inboth the high and low draft conditions.

It will be appreciated that the foregoing described vessel may beemployed in other types of marine operations and fitted out withsuitable structure consonant with such employment. For example, insteadof mounting a drilling rig on platform 38, the column stabilized vesselof this invention may be adapted as a heavy duty derrick barge bymounting a heavy duty crane thereon along the centerline of the vessel,with other portions of the vessel being constructed similarly tocorresponding portions of the vessel herein described and illustrated,with appropriate modification as to size. This vessel can be used as adredge vessel by mounting and operating standard dredging equipmenttaking advantage of all the motion minimizing characteristics of theforegoing described vessel.

It is therefore seen that the objects of the present invention are fullyaccomplished in that the present vessel provides a deep water drillingunit having rapid mobility in transit between drilling sites as well asan exceeding ly stable structure when in high draft condition atdrilling depths over a drill site. Moreover, the 'vessel provides for aself-contained drilling operation or as a tender servicing otherdrilling structures.

This invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof.

What is claimed and desired to be secured by United States LettersPatent is:

1. A variable draft vessel comprising:

a pair of elongated hulls disposed in side-by-side spaced substantiallyparallel relation one to the other;

a working platform;

means connected to said hulls and said platform to support said platformin spaced relation above said hulls; said connecting means including atleast six upstand- 15 ing stabilizing columns disposed on said hulls, apair of said columns being located adjacent opposite ends of said hullson opposite sides of the pitch axis of the vessel, at least threecolumns being located on each hull and on opposite sides of the rollaxis of the vessel;

said hulls having. a combined displacement suflicient to float thevessel in a low draft condition with the hulls having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft conditionand refloat the vessel with the hulls having freeboard, the displacementof the submerged hulls and portions of said connecting means beingsufficient to maintain the vessel buoyant in the high draft condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof and between said platform andsaid hulls;

the cross sectional area and configuration of said columns, the weightdistribution of the vessel, and the geometry of the submerged hulls andportions of said connecting means being such as to maintain the naturalperiod of the vessel in the high draft condition no less than sixseconds;

said vessel having a length to width ratio of at least 2 to l;

said stabilizing columns being connected to said hulls .A variable draftvessel comprising:

pair of elongated hulls disposed in side-by-side spaced substantiallyparallel relation one to the other;

a working platform; means connected to said hulls and said platform tosupport said platform in spaced relation above said hulls; saidconnecting means including at least four stabilizing columns located onsaid hulls with a pair of said columns on each of the opposite sides ofthe respective pitch and roll axes of the vessel;

said hulls having a combined displacement sufficient to float the vesselin a low draft condition with the hulls having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft con- 0dition and refloat the vessel with the hulls having freeboard, thedisplacement of the submerged hulls and portions of said connectingmeans being sufficient to maintain the vessel buoyant in the high draftcondition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof, and between said hulls andsaid platform;

the cross sectional area and configuration of the columns, the weightdistribution of the vessel, and the geometry of the submerged hulls andportions of said connecting means being such as to maintain the naturalperiod of the vessel in the high draft condition no less than 6 seconds;

said stabilizing columns being connected to said hulls on the outboardsides thereof with the distance between the pitch axis and the centroidof the stabilizing columns on opposite sides of the pitch axis beinggreater than the transverse distance between the roll axis to thecentroid of the stabilizing columns on opposite sides of the roll axis,said stabilizing columns having outboard portions thereof which extendtransversely beyond the outboard sides of the respective hulls therebyproviding increased moment for stabilizing action about the roll axis.

3. A variable draft vessel comprising:

a pair of elongated hulls disposed in side-by-side spaced substantiallyparallel relation one to the other;

a working platform;

a plurality of elements upstanding from said hulls with at least aportion of said elements being connected to said platform to support theplatform in spaced relation above the hulls a predetermined height atleast equal to the height of the maximum anticipated wave; at least aportion of said upstanding elements including at least six stabilizingcolumns disposed on said hulls and upstanding therefrom a predeterminedeffective height at least equal to the height of the maximum anticipatedwave, a pair of said columns being located adjacent opposite ends ofsaid hulls on opposite sides of the pitch axis of the vessel, at leastthree columns being located on each hull and on opposite sides of theroll axis of the vessel;

said hulls having a combined displacement sufiicient to float the vesselin a low draft condition with the hulls having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of said elements below the waterlinethereby to provide a high draft condition and refloat the vessel withthe hulls having freeboard, the displacement of the submerged hulls andportions of said elements being sufficient to maintain the vesselbuoyant in the high draft condition;

said elements being spaced one from the other to provide an open framearea therebetween and between said platform and hulls;

the cross sectional area and configuration of the columns, the weightdistribution of the vessel and the geometry of the submerged hulls andportions of said elements being such as to maintain the natural periodof the vessel in the high draft condition no less than 6 seconds;

said vessel having a length to width ratio of at least the displacementof the submerged hulls and portions of said elements being sufficient tomaintain the vessel buoyant in the high draft condition with the meanwaterline located a distance above the hulls at substantially one-halfthe effective height of said stabilizing columns;

said stabilizing columns being connected to said hulls on the outboardsides thereof with the distance between the pitch axis and the centroidof the stabilizing columns on opposite sides of the pitch axis beinggreater than the transverse distance between the roll axis and thecentroid of the stabilizing columns on opposite sides of the roll axis,said stabilizing columns having outboard portions thereof which extendtransversely beyond the outboard sides of the respective hulls therebyproviding increased moment for stabilizing action about the roll axis.

4. A variable draft vessel comprising: no more than two hulls disposedin side-by-side spaced substantially parallel relation one to the other;

a working platform;

means connected to said bulls and said platform to support said platformin spaced relation above said hulls a predetermined height at leastequal to the height of the maximum anticipated wave, said connectingmeans including at least six upstanding stabilizing columns disposed onsaid hulls and upstanding therefrom a predetermined effective height ofat least equal to the height of the maximum anticipated wave, a pair ofsaid columns being located adjacent opposite ends of said hulls onopposite sides of the pitch axis of the vessel, at least three columnsbeing located on each hull and on opposite sides of the roll axis of thevessel.

said hulls having a combined displacement sufiicient to float the vesselin a low draft condition with the hulls having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of the connecting means below saidplatform below the waterline thereby to provide a high draft conditionand refloat the vessel with the hulls having freeboard, the displacementof the submerged hulls and portions of said connecting means beingsufficient to maintain the vessel buoyant in the high draft condition;

said connecting means being spaced one from the other to provide an openframe area between the compoiients thereof and between said platform andsaid hu ls;

said vessel having a length to width ratio of at least said stabilizingcolumns being connected to said hulls on the outboard sides thereof withthe distance between the pitch axis and the centroid of the stabilizingcolumns on opposite sides of the pitch axis being greater than thetransverse distance between the roll axis and the centroid of thestabilizing columns on opposite sides of the roll axis, said stabilizingcolumns having outboard portions thereof which extend transverselybeyond the outboard sides of the respective hulls thereby providingincreased moment for stabilizing action about the roll axis.

5. A variable draft vessel comprising:

no more than two hulls disposed in side-by-side spaced substantiallyparallel relation one to the other;

a working platform;

means connected to said hulls and to said platform to support saidplatform in spaced relation above said hulls a predetermined height atleast equal to the height of the maximum anticipated wave, saidconnecting means including at least six upstanding stabilizing columnsdisposed on said hulls and upstanding therefrom a predeterminedeffective height at least equal to the height of the maximum anticipatedwave, a pair of said columns being located adjacent opposite ends ofsaid hulls on opposite sides of the pitch axis of the vessel, at leastthree columns being located on each hull and on opposite sides of theroll axis of the vessel with the cross sectional area of said columnsbeing substantially constant throughout the effective height thereof;

said hulls having a combined displacement sufficient to float the vesselin a low draft condition with the hulls having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of said connecting means below theWaterline thereby to provide a high draft condition and refloat thevessel with the hulls having freeboard, the displacement of thesubmerged hulls and portions of said connecting means being sufficientto maintain the vessel buoyant in the high draft condition with the meanwaterline located a distance above the hulls at substantially one-halfthe effective height of said stabilizing columns;

said connecting means being spaced one from the other to provide an openframe area between the components thereof and between said platform andsaid hulls;

the cross sectional area and configuration of said columns, the weightdistribution of the vessel and the geometry of the submerged hulls andportions of said connecting means being such as to maintain the naturalperiod of the vessel in the high draft condition no less than 6 seconds;

said vessel having a length to width ratio of at least said stabilizingcolumns being connected to said hulls on the outboard sides thereof withthe distance between the pitch axis and the centroid of the stabilizingcolumns on opposite sides of the pitch axis being greater than thetransverse distance between the roll axis and the centroid of thestabilizing columns on opposite sides of the roll axis, said stabilizingcolumns having outboard portions thereof which extend transverselybeyond the outboard sides of the respective hulls thereby providingincreased moment for stabilizing action about the roll axis.

6. A variable draft vessel comprising:

a base structure having hydrostatic and hydrodynamic characteristics;

a working platform;

a plurality of elements upstanding from said base structure with atleast a portion of said elements being connected to said platform tosupport said platform in spaced relation about said base structure, atleast a portion of said upstanding elements including at least sixstabilizing columns disposed on said base structure with a pair of saidcolumns being located adjacent opposite ends of said base structure onopposite sides of the pitch axis of the vessel, at least three columnsbeing located on said base structure on opposite sides of the roll axisof the vessel;

said base structure having displacement suflicient to float the vesselin a low draft condition with the base structure having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the base structure and portions of said elements below thewaterline thereby to provide a high draft condition and refloat thevessel with the base structure having freeboard;

said elements being spaced one from the other to provide an open framearea therebetween and between said platform and said hulls;

the cross sectional areas and configuration of said columns, the weightdistribution of the vessel, and the geometry of the submerged basestructure and portions of said elements being such as to maintain thenatural period of the vessel in the high draft condition no less than 6seconds;

said vessel having a length to width ratio of at least said stabilizingcolumns being connected to said base structure on the outboard sidesthereof with the distance between the pitch axis and the centroid of thestabilizing columns on opposite sides of the pitch axis being greaterthan the transverse distance between the roll axis and the centroid ofthe stabilizing columns on opposite sides of the roll axis, saidstabilizing columns having outboard portions thereof which extendtransversely beyond the outboard sides of the base structure therebyproviding increased moment for stabilizing action about the roll axis.

7. A variable draft vessel comprising:

a pair of elongated flotation members disposed in sideby-side spacedsubstantiall parallel relation one to the other;

a working platform;

means connected to said flotation members and said platform to supportsaid platform in fixed spaced relation above said members;

said connecting means including at least six upstanding stabilizingcolumns disposed on said members, a pair of said columns being locatedadjacent opposite ends 119 of said members on opposite sides of thepitch axis of the vessel, at least three columns being located on eachmember and on opposite sides of the roll axis of the vessel;

said platform being spaced above said members a predetermined height atleast equal to the height of the maximum anticipated wave, said columnsupstanding from said members a predetermined effective height at leastequal to the height of the maximum anticipated wave, the cross sectionalarea of said columns being substantially constant throughout theeffective height thereof;

said members having a combined displacement sufficient to float thevessel in a low draft condition with the members having freeboard;

means for ballasting and deballasting the vessel to respectivelysubmerge the members and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return the vessel to the low draftcondition with the members having freeboard, the displacement of thesubmerged members and portions of said connecting means being sufficientto maintain the vessel buoyant in the high draft floating condition;

said connecting means being spaced one from the other to provide an openframe area between the compo nents thereof and between said platform andsaid members;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition; the cross sectional area and configuration of saidcolumns, the weight distribution of the vessel, and the geometry of thesubmerged members and portions of said connecting means being such thatthe vessel obtains motion minimizing characteristics in the high draftcondition; and means carried by the vessel for conducting drillingoperations while the vessel is maintained in the high draft floatingcondition; 7

said vessel having a length to width ratio of about 8. A vesselaccording to claim 7 wherein the natural periods in pitch and roll ofthe vessel in the high draft condition lie within a range of 6 to 40seconds.

9. A vessel according to claim 7 wherein the displacement of saidsubmerged members and portions of said connecting means is sufiicient tomaintain the vessel buoyant in the high draft condition with the meanwaterline located a distance above said members at substantiallyone-half the effective height of said stabilizing colurns;

said ballasting and deballasting means being operable to vary thesubmergence of the vessel when in the high draft condition such that thedistance between the mean waterline and either the underside of theplatform or the topside of said members is not less than 0.75 of themean wave height for wave heights less than the maximum anticipated waveto preclude amplification of vessel motion due to interaction of waveand vessel motion.

10. A vessel according to claim 7 including a stabilizing column on eachmember located adjacent the transverse axis through the center offlotation of the vessel, said connecting means including a plurality ofsupport members forming trusses extending between said member and saidplatform at spaced longitudinal positions along the vessel, said trussesincluding transversely extending elements connecting one member to theother with said elements lying above the uppermost portions of saidflotation members.

11. A vessel according to claim 7 wherein at least one of saidstabilizing columns on each said member has a cross section elongated ina direction substantially parallel to the roll axis of the vessel.

12. A vessel according to claim 7 wherein at least one of saidstabilizing columns on each said member is substantially oblong in crosssection and extends longitudinally parallel to said member a distancegreater than its width.

13. A vessel according to claim 7 wherein said drilling means includes adrilling rig mounted on said platform over the space between saidmembers.

14. A vessel according to claim 7 wherein each of said members has bowand stern designs to minimize resistance to movement through the waterin said low draft condition.

15. A vessel according to claim 7 wherein the centroids of said columnsare transversely spaced from vertical planes containing the longitudinalcenterlines of said members.

16. A vessel according to claim 7 wherein the centroids of said columnsare spaced outboard of vertical planes passing through the longitudinalcenterlines of said members.

17. A vessel according to claim 7 including a crane for transferringloads between horizontally spaced positions.

18. A vessel according to claim 7 wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member to alter thenatural period of said vessel.

19. A vessel according to claim 18 wherein said ballasting anddeballasting means includes at least four ballast compartments in eachof said members, a pair of said ballast compartments in each memberbeing transversely spaced one from the other, another pair of saidballast compartments in each member being longitudinally spaced one fromthe other and means for effecting ballast transfer into and out of theballast compartments of each said pair thereof to alter the naturalperiod of said vessel about roll and pitch axes.

20. A variable draft vessel comprising: a pair of elongated hullsdisposed in side-by-side spaced substantially parallel relation one tothe other;

a working platform;

means connected to said hulls and said platform to support said platformin fixed spaced relation above said hulls;

said connecting means including at least four stabilizing columnslocated on said hulls with a pair of said columns on each of theopposite sides of the respective pitch and roll axes of the vessel;

the cross sectional area of said columns being substantially constantthroughout the height thereof; said hulls having combined displacementsufficient to float the vessel in a low draft condition with the hullshaving freeboard, each of sad hulls havng bow and stern designs tominimize resistance to movement through the water in said low draftcondition;

means for ballasting and deballasting the vessel to respectivelysubmerge the bulls and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return the vessel to the low draftcondition with the hulls having free board, the displacement of thesubmerged hulls and portions of said connecting means being sufficientto maintain the vessel buoyant in the high draft floating condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof, and between said hulls andsaid platform;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

the cross sectional area and configuration of the columns, the weightdistribution of the vessel, and the geometry of the submerged hulls andportions of said connecting means being such that the vessel obtainsmotion minimizing characteristics in the high draft condition;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition.

21. A vessel according to claim 20 wherein said connecting meansincludes at least 6 upstanding stabilizing columns disposed on saidhulls, a pair of said columns being located adjacent opposite ends ofsaid hulls on the opposite sides of the pitch axis of the vessel, atleast 3 columns being located on each hull and on opposite sides of theroll axis of the vessel.

22. A vessel according to claim 20 having a length to width ratio of atleast 2 to 1.

23. A vessel according to claim 20 wherein said platform is spaced abovesaid hulls a predetermined height at least equal to the height of themaximum anticipated wave, said columns upstanding from said hulls apredetermined effective height at least equal to the height of themaximum anticipated wave.

24. A vessel according to claim 20 wherein the natural periods in pitchand roll of the vessel in the high draft condition lies within a rangeof 6 to 40 seconds.

25. A vessel according to claim 23 wherein the displacement of saidsubmerged hulls and portions of said connecting means is suflicient tomaintain the vessel buoyant in the high draft condition with the meanwaterline located a distance above said hulls at substantially one-halfthe effective height of said stabilizing columns;

said ballasting and deballasting means being operable to vary thesubmergence of the vessel when in the high draft condition such that thedistance between the mean waterline and either the underside of theplatform or the topside of said hulls is not less than 0.75 of the meanwave height to preclude amplification of vessel motion due tointeraction of Wave and vessel motion.

26. A vessel according to claim 20 wherein said drilling means includesa drilling rig mounted on said platform over the space between thehulls.

27. A vessel according to claim 24 wherein said ballasting anddeballasting means include a plurality of compartments in each of saidhulls together with means for selectively ballasting and deballastingsaid compartments.

28. A vessel according to claim 20 wherein the centroids of said columnsare transversely spaced from vertical planes passing through thelongitudinal centerlines of said hulls.

29. A vessel according to claim 20 wherein said platform is spaced abovesaid hulls a predetermined effective height at least equal to the heightof the maximum anticipated wave, said columns upstanding from said hullsa predetermined effective height at least equal to the height of themaximum anticipated wave, said ballasting and deballasting meansincluding a plurality of compartments in each of said hulls togetherwith means for selectively ballasting and deballasting saidcompartments, the displacement of the submerged hulls and portions ofsaid connecting means being suflicient to maintain the vessel buoyant inthe high draft condition with the mean waterline located a distanceabove the hulls at substantially one-half the effective height of saidstabilizing columns.

30. A vessel according to claim 20 wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid hulls, said two ballast compartments being horizontally spaced onefrom the other, and means for selectively ballasting and deballastingsaid 22 two compartments in each said hull to alter the natural periodof said vessel.

31. A variable draft vessel comprising: a pair of elongated flotationmembers disposed in side-by-side spaced substantially parallel relationone to the other;

a working platform;

a plurality of elements upstanding from said members with at least aportion of said elements being connected to said platform to support theplatform in spaced relation above the members; at least a portion ofsaid upstanding elements including at least six stabilizing columnsdisposed on said members and upstanding therefrom a predeterminedheight, a pair of said columns being located adjacent opposite ends ofsaid members on opposite sides of the pitch axis of the vessel, at leastthree columns being located on each member and on opposite sides of theroll axis of the vessel;

the cross sectional area of said columns being substantially constantthroughout the predetermined height thereof;

said flotation members having a combined displacement sufficient tofloat the vessel in a low draft condition;

means for ballasting and deballasting the vessel to respectivelysubmerge the members and portions of said elements below the waterlinethereby to provide a high draft, otherwise unsupported, floatingcondition and return the vessel to a low draft condition; thedisplacement of the submerged members and portions of said elementsbeing suflicient to maintain the vessel buoyant in the high draftfloating condition;

said elements being spaced one from the other to provide an open framearea therebetween and between said platform and members;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

the cross sectional area and configuration of the columns, the weightdistribution of the vessel and the geometry of the submerged members andportions of said elements being such that the vessel obtains motionminimizing characteristics in the high draft condition;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition;

said vessel having a length to width ratio of at least 32. A vesselaccording to claim 31 including a crane for transferring loads betweenhorizontally spaced positions.

33. A vessel according to claim 31 wherein the displacement of thesubmerged members and portions of said elements is sufficient tomaintain the vessel buoyant in the high draft condition with the meanwaterline located a distance above the members at substantially one-halfthe predetermined height of said stabilizing columns.

34. A vessel according to claim 31 wherein the natural periods in pitchand roll of the vessel in the high draft condition lie within a range of6 to 40 seconds.

35. A vessel according to claim 33 wherein said columns upstand fromsaid members a predetermined height at least equal tothe height of themaximum anticipated wave; said ballasting and deballasting means beingoperable to vary the submergence of the vessel when in the high draftcondition such that the distance between the mean waterline and eitherthe underside of the platform or the topside of said members is not lessthan 0.75 of the mean wave height for wave heights less than the maximumanticipated wave to preclude amplification of vessel motion due tointeraction of wave and vessel motion.

36. A vessel according to claim 33 including a stabilizing column oneach member located adjacent the transverse axis through the center offlotation of the vessel, said connecting means including a plurality oftrusses extending between said members and said platform at spacedlongitudinal positions along the vessel, said trusses includingtransversely extending elements connecting one member to the other withsaid elements lying adjacent the uppermost portions of said members.

37. A vessel according to claim 31 wherein at least one of saidstabilizing columns on each said member has a cross section elongated ina direction substantially parallel to the roll axis of the vessel.

38. A vessel according to claim 37 wherein the centroid of said onestabilizing column on each said member is spaced outboard of thevertical plane containing the longitudinal centerline of thecorresponding member.

39. A vessel according to claim 31 wherein the members have freeboard inthe low draft condition of the vessel.

40. A vessel according to claim 31 wherein said platform is fixed inspaced relation above said members each of said members having bow andstem designs to minimize resistance to movement through the water insaid low draft condition.

41. A vessel according to claim 31 wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member to alter thenatural period of said vessel.

42. A variable draft vessel comprising: no more than two hulls disposedin side-by-side spaced substantially parallel relation one to the other;

a working platform;

means connected to said hulls and said platform to support said platformin spaced relation above said hulls, said connecting means including atleast six upstanding stabilizing columns disposed on said hulls andupstanding therefrom a predetermined effective height, a pair of saidcolumns being located adjacent opposite ends of said hulls on oppositesides of the pitch axis of the vessel, at least three columns beinglocated on each hull and on opposite sides of the roll axis of thevessel;

said hulls having a combined displacement suflicient to float the vesselin a low draft condition with the hulls having freeboard;

the cross sectional area of said columns being substantially constantthroughout the effective height thereof;

means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of the connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return the vessel to the low draftcondition with the hulls having freeboard, the displacement of thesubmerged hulls and portions of said connecting means being sufficientto maintain the vessel buoyant in the high draft floating condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof and between said platform andsaid hulls;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition;

said vessel having a length to width ratio of at least 43. A vesselaccording to claim 42 including a crane for transferring loads betweenhorizontally spaced positrons.

44. A vessel according to claim 42 wherein the displacement of thesubmerged hulls and portions of said connecting means is suflicient tomaintain the vessel in the high draft condition with the mean waterlinelocated a distance above the hulls at substantially one-half theeffective height of said stabilizing column-s.

45. A vessel according to claim 42 wherein the cross sectional area andconfiguration of the columns, the weight distribution of the vessel andthe geometry of the submerged hulls and portions of said connectingmeans are such that the vessel obtains a natural period in the highdraft condition no less than 6 seconds.

46. A vessel according to claim 44 wherein said ballasting anddeballasting means is operable to vary the submergence of the vesselwhen in the high draft condition such that the distance between the meanwaterline and either the underside of the platform or the topside ofsaid hulls is not less than 0.75 of the mean wave height for waveheights less than the maximum anticipated wave to preclude amplificationof vessel motion due to inter action of wave and vessel motion.

47. A vessel according to claim 42 wherein said stabilizing columns aresubstantially oblong in cross section and extend longitudinally parallelto said hulls a distance greater than their width.

48. A vessel according to claim 42 wherein said drilling means includesa drilling rig mounted on said platform over the space between thehulls.

49. A vessel according to claim 42 wherein said platform is fixed inspaced relation above said hulls, each of said hulls having bow andstern designs to minimize resistance to movement through the water insaid low draft condition.

50. A vessel according to claim 42 wherein saidballasting anddeballasting means includes at least two ballast compartments in each ofsaid hulls, said two ballast compartments being horizontally spaced onefrom the other, and means for selectively ballasting and deballastingsaid two compartments in each said hull to alter the natural period ofsaid vessel.

51. A vessel according to claim 42 wherein the cross sectional area andconfiguration of said columns, the weight distribution of the vessel andthe geometry of the submerged hulls and portions of said connectingmeans being such as to provide motion minimizing characteristics to thevessel.

52. A variable draft vessel comprising: a base structure havinghydrostatic and hydrodrynamic characteristics;

a working platform;

a plurality of elements carried by said base structure with at least aportion of said elements upstanding therefrom and being connected tosaid platform to support said platform in spaced relation above saidbase srt-ucture, at least a portion of said elements including at leastfour stabilizing columns carried by said base structure and extendingvertically a predetermined effective height between generally horizontalplanes containing said platform and said base structure, a pair of saidcolumns being located adjacent opposite ends of said base structure onopposite sides of the pitch axis of the vessel, at least two columnsbeing located on opposite sides of the roll axis of the vessel;

the cross sectional area of said columns being substantially constantthroughout the effective height thereof;

said base structure having a displacement sufiicient to float the vesselin a low draft condition with the base structure having freeboard;

means for ballasting and deballasting the vessel to respectivelysubmerge the base structure and portions of said elements below thewaterline thereby to provide a high draft, otherwise unsupported,floating condition and return the vessel to the low draft condition withthe base structure having freeboard;

said elements being spaced one from the other to provide an open framearea therebetween and between said platform and said base structure;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

the cross sectional areas and configuration of said columns, the weightdistribution of the vessel, and the geometry of the submerged basestructure and portions of said elements being such that the vesselobtains a natural period in the high draft condition no less than 6seconds;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition;

said vessel having a length to width ratio of at least 2 53. A vesselaccording to claim 52 including a crane for transferring loads betweenhorizontally spaced positions.

54. A vessel according to claim 52 wherein said platform is spaced abovesaid base structure a predetermined height at least equal to the heightof the maximum anticipated wave, said columns extending vertically apredetermined eflective height at least equal to the height of themaximum anticipated Wave.

55. A vessel according to claim 52 wherein the natural periods in pitchand roll of the vessel lie within a range of 6 to 40 seconds.

56. A vessel according to claim 52 wherein said ballasting anddeballasting means includes at least two ballast compartments carried bysaid base structure on each of the opposite sides of the centerline ofsaid vessel, said two ballast compartments being horizontally spaced onefrom the other, and means for effecting ballast transfer into and out ofsaid two compartments on each side of said vessel to alter the naturalperiod of said vessel.

57. A vessel according to claim 52 wherein at least one column on eachof the opposite sides of the roll axis is oblong with its long axisextending substantially parallel to the longitudinal centerline of saidvessel.

58. A variable draft column stabilized vessel comprising: a pair ofelongated flotation members disposed in side-by-side spacedsubstantially parallel relation one to the other;

a working platform;

means connected to said members and said platform to support saidplatform in fixed spaced relation above said members;

said connecting means including at least six stabilizing columnsdisposed on said members and upstanding therefrom a predeterminedheight;

a pair of said columns being located adjacent opposite ends of saidmembers on opposite sides of the pitch axis of said vessel, at leastthree columns being located on each member and on opposite sides of theroll axis of said vessel, at least one column on each member beingsubstantially oblong in cross section and disposed such that its longaxis extends substantially parallel to the longitudinal axes of themembers;

the cross sectional area of said columns being substantially constantthroughout the height thereof;

said members having a combined displacement sufficient to float thevessel in a low draft condition with the members having freeboard;

means for ballasting and deballasting said vessel to respectivelysubmerge the members and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return said vessel to the low draftcondition with the members having freeboard, the displacement of thesubmerged members and portions of said connecting means being suflicientto maintain said vessel buoyant in the high draft floating condition;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof and between said platform andsaid members;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition.

59. A vessel according to claim 58 including a crane for transferringloads between horizontally spaced positions.

'60. A vessel according to claim 58 wherein the cross sectional area andconfiguration of said columns, the weight distribution of the vessel andthe geometry of the submerged members and portions of said connectingmeans are such as to provide motion minimizing characteristics to saidvessel in roll, pitch and heave in the high draft condition.

61. The vessel according to claim 58 wherein said members are connectedone to the other by said connecting means with the area between saidmembers below a horizontal plane extending through said members adjacent their upper surfaces being open and substantially unobstructed.

62. A variable draft vessel according to claim 58 wherein said platformis supported in spaced relation above said members a predeterminedheight at least equal to the height of the maximum anticipated wave,said columns upstanding from said members an effective height at leastequal to the height of the maximum anticipated wave, the displacement ofsaid submerged members and portions of said connecting means beingsufiicient to maintain said vessel buoyant in the high draft conditionwith the mean waterline located a distance above said members atsubstantially one-half the effective height of said stabilizing columns.

63. A vessel according to claim 58 wherein said connecting meansincludes a plurality of support members forming trusses extendingbetween said members and said platform at spaced longitudinal positionsalong said vessel, said trusses including transversely extending memberslying adjacent the uppermost portions of said members.

64. A vessel according to claim '58 wherein each of said members hasstreamlined bow and stern portions to minimize resistance to movementthrough the water in said low draft condition.

65. A vessel according to claim 58- wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member.

66. A vessel according to claim 58 wherein said drilling means includesa drilling mast disposed substantially along the longitudinal centerlineof the vessel for supporting a drill string extending between saidmembers.

67. A vessel according to claim 58 wherein said vessel has a length towidth ratio of at least 2 to l.

68. A vessel according to claim 58 wherein said platform is supported inspaced relation above said members a predetermined height at least equalto the height of the maximum anticipated wave, said columns upstandingfrom said members an effective height at least equal to the height ofthe maximum anticipated wave, the displacement of said submerged membersand portions of said connecting means being sufficient to maintain saidvessel buoyant in the high draft condition with the mean waterlinelocated a distance above said members at substantially one-half theeffective height of said stabilizing columns, said connecting meansincluding a plurality of support members forming trusses extendingbetween said members and said platform at spaced longitudinal positionsalong said vessel, said trusses including transversely extending membersconnecting one flotation member to the other and lying adjacent theuppermost portions of said flotation members, said ballasting anddeballasting means including at least two ballast compartments in eachof said flotation members, said two ballast compartments beinghorizontally spaced one from the other, and means for selectivelyballasting and deballasting the compartments in each said member.

69. A vessel according to claim 58 wherein the centroids of said columnsare spaced outboard of vertical planes containing the longitudinal axesof said members.

70. A variable draft vessel comprising: a pair of elongated flotationmembers disposed in side-by-side spaced substantially parallel relationone to the other;

a working platform;

means connected to said members and said platform to support saidplatform in fixed spaced relation above said members; said connectingmeans including at least six upstanding stabilizing columns disposed onsaid members, a pair of said columns being located adjacent oppositeends of said members on opposite side of the pitch axis of the vessel,at least three columns being located on each member and on oppositesides of the roll axis of the vessel, the centroids of said columnsbeing spaced outboard of vertical planes containing the longitudinalaxes of said members; the cross sectional area of said columns beingsubstantially constant throughout the height thereof;

said members having a combined displacement suflicient to float thevessel in a low draft condition with the members having freeboard;

means for ballasting and deballasting the vessel to respectivelysubmerge the members and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return the vessel to the low draftcondition with the members having freeboard, the displacement of thesubmerged members and portions of said connecting means being suflicientto maintain the vessel buoyant in the high draft floating condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof and between said platform andsaid members;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

the cross sectional area and configuration of said columns, the weightdistribution of the vessel, and the geometry of the submerged membersand portions of said connecting means being such as to provide motionminimizing characteristics to said vessel in roll, pitch and heave inthe high draft condition;

and means carried by the vessel for conducting drilling operations whilethe vessel is maintained in the high draft floating condition.

71. A vessel according to claim 70 including a crane for transferringloads between horizontally spaced positions.

72. A vessel according to claim 70 wherein the cross sectional area ofsaid columns is substantially constant throughout the predeterminedheight thereof.

73. A variable draft vessel according to claim 70 wherein said platformis supported in spaced relation above said members a predeterminedheight at least equal to the height of the maximum anticipated wave,said columns upstanding from said members an effective height at leastequal to the height of the maximum anticipated wave, the displacement ofsaid submerged members and portions of said connecting means beingsuflicient to maintain said vessel buoyant in the high draft conditionwith the mean waterline located a distance above said members atsubstantially one-half the effective height of said stabilizing columns.

74. A vessel according to claim 70 wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member.

75. A vessel according to claim 70 wherein said drilling means includesa drilling mast disposed substantially along the longitudinal centerlineof the vessel for supporting a drill string extending between saidmembers.

76. A vessel according to claim 70 wherein each of said members hasstreamlined bow portions to minimize resistance to movement through thewater in said low draft condition.

77. A vessel according to claim 70 wherein said platform is supported inspaced relation above said members a predetermined height at least equalto the height of the maximum anticipated wave, said columns upstandingfrom said members an effective height at least equal to the height ofthe maximum anticipated wave, the displacement of said submerged membersand portions of said connecting means being suflicient to maintain saidvessel buoyant in the high draft condition with the mean waterlinelocated a distance above said members at substantially one-half theeffective height of said stabilizing columns, said ballasting anddeballasting means including at least two ballast compartments in eachof said members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member, each of saidmembers having streamlined bow portions to minimize resistance tomovement throu h the Water in said low draft condition.

78. A variable draft vessel comprising:

a pair of discrete elongated flotation members disposed in side-by-sidespaced substantially parallel relation one to the other;

a working platform;

means for supporting said platform in fixed spaced relation above saidmembers;

said connecting means including at least six stabilizing columnsdisposed on said members and upstanding therefrom, a pair of said columsbeing located adjacent opposite ends of said members on opposite sidesof the pitch axis of said vessel, at least three columns being locatedon each member and on opposite sides of the roll axis of said vessel;

the cross sectional area of said columns being substantially constantthroughout the height thereof;

at least one of said columns on each said member being substantiallyoblong in cross section and disposed such that its long axis extendssubstantially parallel to the longitudinal axes of said members;

means for ballasting and deballasting said vessel to selectively obtainhigh and low draft, otherwise unsupported, floating conditions with saidmembers and portions of said connecting means below said platformlocated below the waterline in the high draft floating condition, thedisplacement of said members and portions of said connecting means beingsufficient to maintain said vessel buoyant in the high draft floatingcondition;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition; and

means carried by the vessel for conducting drilling operations while thevessel is maintained in the high draft floating condition.

79. A variable draft vessel comprising:

a pair of discrete elongated flotation members disposed in side-by-sidespaced substantially parallel relation one to the other;

a working platform;

means for supporting said platform in fixed spaced relation above saidmembers;

said connecting means including at least six stabilizing columnsdisposed on said members and upstanding therefrom, a pair of saidcolumns being located adjacent opposite ends of said members on oppositesides of the pitch axis of said vessel, at least three columns beinglocated on each member and on opposite sides of the roll axis of saidvessel;

at least one of said columns on each said member being substantiallyoblong in cross section and disposed such that its long axis extendssubstantially parallel to the longitudinal axes of said members;

and means for ballasting and deballasting said vessel to selectivelyobtain high and low draft conditions with said members and portions ofsaid connecting means below said platform located below the waterline inthe high draft condition, the displacement of said members and portionsof said connecting means being sufiicient to maintain said vesselbuoyant in the high draft condition;

said columns being substantially constant in cross sectional areathroughout their height, said vessel having a length to width ratiogreater than 2 to 1.

80. The vessel according to claim 79 wherein the centroid of said oblongcolumn on each said member lies outboard of vertical planes passingthrough the longitudinal centerlines of said member.

81. The vessel according to claim 79 wherein said ballasting anddeballasting means includes at least two ballast compartments in each ofsaid members, said two ballast compartments being horizontally spacedone from the other, and means for selectively ballasting anddeballasting said two compartments in each said member.

82. The vessel according to claim 79 including a derrick.

83. A variable draft vessel comprising:

a base structure having hydrostatic and hydrodynamic characteristics;

a working platform;

a plurality of elements carried by said base structure with at least aportion of said elements upstanding therefrom and being connected tosaid platform to support said platform in spaced relation above saidbase structure, at least a portion of said elements including at leastfour stabilizing columns carried by said base structure and extendingvertically a predetermined effective height between generally horizontalplanes containing said platform and said base structure, a pair of saidcolumns being located adjacent opposite ends of said base structure onopposite sides of the pitch axis of the vessel, at least two columnsbeing located on opposite sides of the roll axis of the vessel;

said base structure having displacement sufficient to float the vesselin a low draft condition with the base structure having freeboard;

and means for ballasting and deballasting the vessel to respectivelysubmerge the base structure and portions of said elements below thewaterline thereby to provide a high draft condition and return thevessel with the base structure having freeboard;

said elements being spaced one from the other to provide an open framearea therebetween and between said platform and said base structure;

the cross sectional areas and configuration of said columns, the weightdistribution of the vessel and the geometry of the submerged basestructure and portions of said elements being such that the vesselobtains a natural period in the high draft condition no less than 6seconds;

said vessel having a length to width ratio of at least 2 to 1; I

said stabilizing column having portions thereof which extendtransversely beyond the outboard sides of said base structure therebyproviding increased moment for stabilizing action about the roll axis.

84. A variable draft vessel comprising:

a pair of elongated hulls disposed in side-by-side spaced substantiallyparallel relation one to the other;

a working platform;

means connected to said hulls and said platform to support said platformin fixed spaced relation above said hulls;

said connecting means including at least four stabilizing columnslocated on said hulls with a pair of said columns on each of theopposite sides of the respective pitch and roll axes of the vessel;

said platform being spaced above said hulls a predetermined height atleast equal to the height of the maximum anticipated wave, said columnsupstanding from said hulls a predetermined effective height at leastequal to the height of the maximum anticipated wave, the cross sectionalarea of said columns being substantially constant throughout theeffective height thereof;

said hulls having a combined displacement sufficient to float the vesselin a low draft condition with the hulls having freeboard, each of saidhulls having bow and stern designs to minimize resistance to movementthrough the water in said low draft condition;

means for ballasting and deballasting the vessel to respectivelysubmerge the hulls and portions of said connecting means below saidplatform below the waterline thereby to provide a high draft, otherwiseunsupported, floating condition and return the vessel to the low draftcondition with the hulls having freeboard, the displacement of thesubmerged hulls and portions of said connecting means being sufficientto maintain the vessel buoyant in the high draft floating condition;

said connecting means being spaced one from the other to provide an openframe area between the components thereof, and between said hulls andsaid platform;

the combined water plane areas of said columns forming at least themajor portion of the total water plane area of said vessel in the highdraft condition;

the cross sectional area and configuration of the columns, the weightdistribution of the vessel, and the geometry of the submerged hulls andportions of said connecting means being such that the vessel obtainsmotion minimizing characteristics in the high draft condition; and

means carried by the vessel for conducting drilling operations while thevessel is maintained in the high draft floating condition.

References Cited UNITED STATES PATENTS TRYGVE M. BLIX, Primary Examiner

